1. Field of the Invention
[The invention I]
The present invention I relates to an information recording and reproducing apparatus with a new system that can be used in an optical disk for recording or for reproducing information and suppresses the generation of light quantity fluctuations.
[The invention II]
The present invention II relates to an information recording and reproducing apparatus having high efficiency in which, in an optical disk for recording or reproducing information, the information is reproduced with a plurality of light beams to increase the transfer speed of a reproducing signal, and the information is recorded with one light beam to increase recording efficiency.
In addition, the present invention II relates to an information recording and reproducing apparatus having high efficiency in which, the information is reproduced by dividing a light beam for use in a stable tracking servo signal such as a three-beam tracking or a DPP (Differential Push-pull) tracking, and the information is recorded with one light beam to increase recording efficiency.
2. Description of the Prior Art
[The invention I]
Conventional optical disks have recorded or reproduced information by using a light beam polarization-branching member, that is located between a radiation light source and a light beam convergence member and separates an outward light beam and a return light beam by polarization of light, in order to increase light efficiency.
In the following, a conventional example that has been used frequently is explained with reference to FIG. 6.
A light beam 602 emitted from a radiation light source 601 passes through a polarization-branching member 603 and a xc2xc wavelength plate 604. The polarization-branching member 603 transmits only a linearly polarized light (P polarized light). The light beam passing through the xc2xc wavelength plate 604 is converted into circularly polarized light and reaches a light beam convergence member 605. The light beam converged by the light beam convergence member 605 reaches a record information carrier 606. The light beam is reflected by the record information carrier 606, passes through the light beam convergence member 605 again and then reaches the xc2xc wavelength plate 604. The light beam becomes a linearly polarized light (S polarized light) perpendicular to the incident light in the xc2xc wavelength plate 604, and is reflected by the polarization-branching member 603. The light beam becomes a transmitted light and a partially diffracted light by a hologram 607 for detection, and reaches a photo detector 608 for reproducing a servo signal or an information signal. Since the effects of the hologram 607 for detection and the photo detector 608 are not directly related to the present invention, an explanation here is omitted.
Since this optical system uses the polarization, when there is an optical element that disturbs the polarization state of a light beam, a light quantity of the light beam returning to the photo detector 608 may decrease. For example, in a record information carrier such as an optical disk made of plastic materials, the molecular structure of the plastics changes due to later stress such as a residual stress from molding and a temperature change, generating larger birefringence. When the record information carrier 606 is birefringent, the polarization state may be disturbed as is mentioned above. In other words, light that is circularly polarized may become elliptically polarized, linearly polarized or leading to a reverse circular polarization, in an extreme case. Accordingly, the light beam that is reflected by the information carrier and transmitted by the xc2xc wavelength plate 604 may be the same polarized light (P polarized light) as the incident beam, in the worst case, and not reflected by the polarization-branching member 603 because the polarization-branching member 603 only transmits P polarized lights. As a result, the light does not reach the photo detector 608 at all. Usually, when a light quantity of a light beam shining into the photo detector 608 decreases, it is designed that an automatic gain control (AGC) circuit is activated to compensate for the decrease of the light quantity. However, when the light quantity is too small, the circuit is, of course, not activated.
As is described above, in reproducing a highly birefringent optical disk, a considerable decrease of a received light quantity has been a problem when separating the outward and return beams by using polarization. Therefore, the conventional detection system poses a problem in that disturbed polarization generates a fluctuation of received light quantity, thereby affecting the detection of a servo signal and an information signal.
[The invention II]
Conventional optical disks have used a light beam emitted from a radiation light source as a single light beam, instead of dividing the light beam into a plurality of light beams, in order to increase a light efficiency in recording. On the other hand, in reproducing optical disks, a plurality of light beams are used in order to stabilize a tracking servo for tracking the disk.
Also, a reproducing method using a plurality of light beams in order to increase the transfer rate of a reproducing signal has already come into an actual use.
In the following, a conventional example of the reproducing system that has been used frequently is explained with reference to FIGS. 14 and 15.
FIG. 14 shows an information recording and reproducing apparatus provided with a system of generating three light beams, reproducing a signal with the light beam in the center and stabilizing a tracking with the light beams on both sides.
A light beam 1702 emitted from a radiation light source (a semiconductor laser) 1701 is divided into three light beams, that is, a zeroth-order light beam, + first order light beam and xe2x88x92 first order light beam, by a diffraction grating 1703, reflected by a beam splitter 1704 and reaches a light beam convergence member (an objective lens) 1705. The light beams converged by the light beam convergence member 1705 reaches a record information carrier 1706. The light beams reflected by the record information carrier 1706 pass through the light beam convergence member 1705 and the beam splitter 1704 again, and then reaches a photo detector 1708 for reproducing a servo signal or an information signal. The three light beams divided by the diffraction grating 1703, which are a zeroth-order light beam, + first order light beam and xe2x88x92 first order light beam, reach corresponding photo detectors 1708, 1709 and 1710. The information signal is obtained from the photo detector 1708, and the signal for tracking servo is obtained from a difference signal of the photo detectors 1709 and 1710. This system is called a three-beam system and widely known in general. Therefore, a detailed explanation is omitted here.
A so-called DPP (Differential Push-pull) system can be included in this three-beam system. A DPP (Differential Push-pull) system is a system in which the photo detectors 1709 and 1710 are divided in half respectively, light spots of the light beams shining into the respective photo detector are located between signal tracks to obtain a far field signal so that a difference between the far field signal and the other far field signal obtained by dividing the photo detector 1708 is used for correcting a tracking signal. This system is also widely known, so a detailed explanation is omitted here.
FIG. 15 shows the diffraction grating 1703 in detail. The diffraction grating is formed by providing a phase step with a constant period on one surface of a glass substrate 1801. Numeral 1802 denotes the zeroth-order light beam, numeral 1803 denotes the + first order light beam, and numeral 1804 denotes the xe2x88x92 first order light beam. The light quantity of this zeroth-order light beam 1802 by the diffraction grating is about 50 to 80% of that of the incident light beam. Therefore, a laser with larger emitting power is needed for recording and reproducing information, leading to cost increase.
These systems share a problem. Namely, although they have no major problem when limited to reproducing information, they suffer from a power shortage of a light source when recording information.
This power shortage is caused by dividing a beam at the time of reproducing. For example, in the three-beam system, a light quantity proportion of the central zeroth-order light beam, + first order light beam, xe2x88x92 first order light beam and a high order diffraction loss is usually about 7:1:1:1. In other words, the light quantity of the central zeroth-order light beam is reduced by approximately 30% compared with that of the incident light beam.
In multi-beam reproducing, since seven light beams, for example, are evenly divided, recording with this system is almost impossible.
[The invention I]
It is an object of the present invention I to prevent a considerable decrease of a received light quantity in recording or reproducing a highly birefringent optical disk for recording or reproducing information when separating the outward and return beams by using polarization and to provide an apparatus having high efficiency that can be used in an optical disk for recording or reproducing information.
In order to solve the above-mentioned problem, the first information recording and reproducing apparatus according to the present invention I includes a radiation light source, a light beam convergence member receiving a light beam emitted from the radiation light source and focusing the light beam on an information carrier, a light beam branching member that is located between the radiation light source and the light beam convergence member, branching the light beam, a photo detector receiving the light beam branched by the light beam branching member, and a variable wave plate located between the light beam branching member and the light beam convergence member.
With this apparatus, the phase difference of the variable wave plate is changed, thereby changing the phase difference of the light beam shining into the information carrier. The radiation light source can be a laser, an LED, a high intensity arc or the like, and the information carrier can be an optical disk, an optical tape, an optical memory carrier or the like.
A second information recording and reproducing apparatus according to the present invention I includes a radiation light source, a collimator receiving a light beam emitted from the radiation light source and turning the light beam into substantially parallel light, a light beam convergence member receiving the substantially parallel light and focusing the substantially parallel light on an information carrier, a light beam branching member that is located between the radiation light source and the collimator, branching the light beam, a photo detector receiving the light beam branched by the light beam branching member, and a variable wave plate located between the collimator and the light beam convergence member.
This apparatus has an advantage in that using the collimator turns the light beam passing through the variable wave plate into the substantially parallel light, reducing the angle dependence of the variable wave plate.
A third information recording and reproducing apparatus according to the present invention I includes a radiation light source, a light beam convergence member receiving a light beam emitted from the radiation light source and focusing the light beam on an information carrier, a light beam polarization-branching member that is located between the radiation light source and the light beam convergence member, branching the light beam by polarization, a variable wave plate located between the light beam polarization-branching member and the light beam convergence member, a photo detector receiving the light beam branched by the light beam polarization-branching member and outputting a current according to an incident light quantity, and an output level judging member judging whether an output of the photo detector is larger or smaller than a predetermined value. A phase difference of the variable wave plate is switched according to a judgement of the output level judging member.
With this apparatus, the phase difference of the variable wave plate can be changed when a detected signal decreases considerably while reproducing a highly birefringent optical disk or the like.
A fourth information recording and reproducing apparatus according to the present invention I includes a radiation light source, a collimator receiving a light beam emitted from the radiation light source and turning the light beam into substantially parallel light, a light beam convergence member receiving the substantially parallel light and focusing the substantially parallel light on an information carrier, a light beam polarization-branching member that is located between the radiation light source and the collimator, branching the light beam by polarization, a variable wave plate located between the collimator and the light beam convergence member, a photo detector receiving the light beam branched by the light beam polarization-branching member and outputting a current according to an incident light quantity, and an output level judging member judging whether an output of the photo detector is larger or smaller than a predetermined value. A phase difference of the variable wave plate is switched according to a judgement of the output level judging member.
This apparatus has an advantage in that the angle dependence of the variable wave plate in the third apparatus can be reduced, as is described in the second apparatus.
In the third and fourth information recording and reproducing apparatus, it is preferable that the light beam convergence member, the light beam polarization-branching member and the variable wave plate are integrated into a coupling member. With such a configuration, a system that stabilizes a far field tracking called a Collect Far-field can be realized. Japanese Patent No. 2523469 describes this system in detail.
A fifth information recording and reproducing apparatus according to the present invention I includes a first radiation light source, a second radiation light source, a dual wavelength branching member receiving a first light beam and a second light beam emitted from the first radiation light source and the second radiation light source and directing the first and second light beams in a predetermined direction, a light beam convergence member receiving the first and second light beams emitted from the dual wavelength branching member and focusing the first and second light beams on an information carrier, a variable wave plate located between the dual wavelength branching member and the light beam convergence member, a first photo detector and a second photo detector receiving the light beam branched by the dual wavelength branching member and outputting a current according to an incident light quantity, and an output level judging member judging whether an output of the first and second photo detectors is larger or smaller than a predetermined value. A phase difference of the variable wave plate is switched according to a judgement of the output level judging member.
Although the birefringence quantity changes according to wavelength when a substrate is birefringent, this apparatus provides an optimal reproducing for the corresponding wavelength by using the above-mentioned system.
In the first to fifth information recording and reproducing apparatus according to the present invention I, it is preferable that the phase difference of the variable wave plate is switched between substantially (n) times a used wavelength and substantially (nxc2x1xc2xc wavelength) times the used wavelength, wherein n is an integer. Setting the phase difference of the variable wave plate to be substantially (n) times the used wavelength provides the state practically equivalent to the one without wave plate. Accordingly, the light shining into the variable wave plate can maintain the initial polarization state and reaches the information carrier. On the other hand, setting the phase difference of the variable wave plate to be substantially (nxc2x1xc2xc wavelength) times the used wavelength provides the xc2xc wavelength plate, and the light beam emitted from the light beam convergence member becomes circularly polarized.
In the present invention I, a new polarization switching member replaces the xc2xc wavelength plate, in order to suppress large fluctuation of the received light quantity of the photo detector, as is described above. By switching polarization when the light quantity received by the photo detector is small, a polarization state of a light beam returning from the information carrier is switched, thereby increasing the light quantity received by the photo detector. In the present invention I, the variable wave plate as the polarization switching member can be a liquid crystal, a crystal showing an electrooptical effect, an electrostrictive element, a polarizing plate (Polaroid polarizing element), a photoelastic element, a plastic plate or the like as long as it can switch the polarization state, and is not limited specifically. When the received light quantity of the photo detector decreases, the element above is electrically or mechanically switched so as to increase the quantity, thereby preventing a servo operation or an information reproducing operation from becoming unstable. As a result, the information recording and reproducing apparatus can obtain a stable reproducing signal with an excellent S/N.
[The invention II]
It is an object of the present invention II to provide an information recording and reproducing apparatus provided with a system for compensating for the power shortage of the light source at the time of recording as is described above and for securing a compatibility of recording and reproducing that does not affect the precision of an optical system.
In the present invention II, a diffraction grating as a beam dividing member is removed when recording and inserted when reproducing, in order to secure compatibility between recording and reproducing. In order to solve this problem, mechanically removing the diffraction grating can be considered. However, while this is generally useful, the compatibility between recording and reproducing was relatively difficult to secure because of the strict setting precision of the diffraction grating that is required.
In order to secure the setting precision of the diffraction grating, the diffraction grating can be switched electrically, rather than mechanically.
Accordingly, in the present embodiment of the invention II, the diffraction grating is formed by electrically changing the refractive index. For example, a diffraction element is formed with a material that generates the refractive index difference when electricity is applied and does not generate the refractive index difference when no electricity is applied.
A first information recording and reproducing apparatus according to the present invention II includes a radiation light source, a light beam convergence member receiving a light beam emitted from the radiation light source and focusing the light beam on an information carrier, a light beam branching member that is located between the radiation light source and the light beam convergence member, branching the light beam in a first state and not branching the light beam in a second state, and a photo detector receiving the light beam branched by the light beam branching member. With this configuration, a stable tracking or a parallel reproducing of information signals can be performed in the first state, and information can be recorded using light sources with relatively low output in the second state.
It is preferable that the above-mentioned apparatus further includes a collimator that receives the light beam emitted from the radiation light source and turns the light beam into substantially parallel light, and that the light beam convergence member receives the parallel light and focuses the parallel light on the information carrier. Including the collimator turns the incident light beam to the light beam convergence member into the parallel light beam, thereby reducing a position dependence of the light beam convergence member in an optical axis direction.
A second information recording and reproducing apparatus according to the present invention II includes a radiation light source, a light beam convergence member receiving a light beam emitted from the radiation light source and focusing the light beam on an information carrier, a light beam branching member that is located between the radiation light source and the light beam convergence member, branching the light beam into a plurality of light beams in a first state and not branching the light beam in a second state, and a plurality of photo detectors receiving the plurality of light beams branched by the light beam branching member, irradiated on a plurality of information tracks on the information carrier and reflected by the information carrier. With above configuration, the information signals can be reproduced in parallel from a plurality of tracks in the first state, and a single light beam can be used to operate reproducing or recording a single track in the second state.
As is described above, with the present invention II, a light beam from a light source is divided into a plurality of light beams to perform a stable tracking control or to reproduce a plurality of information tracks in parallel, and a single light beam is used to record information, by using common light sources with relatively low output.